Optical storage technology comprises a wide and growing variety of disc and application specifications. Disc specifications include, for example, CD-ROM and DVD-ROM for pre-recorded discs, CD-R, DVD-R, DVD+R, Blu-Ray Disc Recordable (BD-R), etc. for write-once discs, and CD-RW, DVD-RW, DVD-RAM, DVD+RW, BD-RE, etc. for rewritable discs. The disc format specifications generally define the physical characteristics of the disc (e.g., mechanical properties, optical signal characteristics, physical arrangement, writing methods, and testing conditions). Application specifications include DVD-Video for video content, DVD-Audio for audio content, and DVD-VR and DVD+VR for real-time video recording (e.g., in camcorders and personal video recorders [PVRs]).
In many optical disc specifications, an optical disc may comprise two areas, including a user data area and a disc information (lead-in) area. The user data area is generally used to write application data, including video, audio, information tables, file system data, etc. The disc information (lead-in) area generally includes data such as disc size, disc type, disc layout, etc. In some optical disc specifications (e.g., CD-R, DVD-R, DVD+R, BD-R, etc.), the disc generally can be written only once. In other disc specifications or formats (e.g., CD-R/W, DVD+RW, BD-RE, etc.), data may be written to the disc more than once.
Optical storage media generally store data as a sequence (e.g., a continuous track) of “pits” (or “marks”) and “lands” (or “space”) on a data-bearing surface that is made reflective by the application of a metallic layer during manufacture. The “lands” are generally parts of the track that are not pits. Pits in read-only storage media are generally molded into the data bearing surface when the discs are formed. Recordable and re-writable discs are generally produced as blanks, and have only a preformed groove or “pre-groove” (together with a limited amount of embossed data in most cases) included during molding.
Data is stored on recordable or re-writable optical storage media using the same pit-land principle, however the pits are generally added by “burning” a special phase-change material layer applied to the disc substrate. In order to write to a recordable or re-writable disc, an optical pick-up head of an optical disc is generally equipped with a higher power write laser in addition to the read laser. Alternatively, one laser can generally perform both functions by operating at lower power output for read operations, and a range of higher power outputs for write operations.
The amount of power used to burn pit features is critical to the shape of those features. The geometry of the shape of the pit features affects the read-back performance of read-back systems. Recordable and re-writable optical storage media are produced by a wide variety of vendors using different materials. Thus, an appropriate writing power level for one medium may be too high or too low for another medium, even if both media are manufactured in accordance with the same specification. Therefore, most optical storage medium specifications include some facility for a vendor to specify an optimal power level. However, even the vendor specified power level may be suboptimal due to normal process variations in the medium and/or the recording device or operational variations in the recording device. Therefore, most optical storage medium specifications also define one or more areas of the media for performing optimal power calibration (OPC) by writing data at various power levels and reading back the data to determine an optimal writing power level. The amount of space available for OPC operations may be limited, and the OPC operations may need to be run multiple times if data is written to the medium in multiple sessions and/or on using multiple different recording devices on the same medium. Consequently, it is desirable for optical storage medium recording devices to obtain the most accurate calibration data possible.
As described above, data is generally written to an optical storage medium by producing marks and spaces on the disc. Each mark and/or space has a “run-length” (e.g., the data “11100011” comprises a 3T mark, a 3T space, and a 2T mark). Many conventional optical storage formats encode data written to the disc according to a run-length limited code, such that every mark and space has a minimum and/or maximum run-length. Some writing errors, including timing offset errors and amplitude variations, correspond to particular run-length transitions (e.g., a transition from a 3T mark to a 2T space, from a 6T space to a 3T mark, etc).
In an exemplary OPC system, as described in U.S. Patent Application Publication No. 2007-0201331 (the relevant portions of which are incorporated herein by reference), data is written to an optical storage disc (or other optical storage medium) according to a predetermined pattern and using a predetermined variety of power levels, timing offsets, and/or other adjustable writing characteristics. The patterns may be selected, for example, to include a variety of run-length transitions. Thus, the system may determine optimal writing characteristics for each transition type by correlating readback characteristics of data read from the disc with the predetermined pattern data and the known writing characteristics.
Furthermore, conventional methods and devices for reading from an optical storage medium perform timing recovery to synchronize reading operations with the data read back from the medium. Timing recovery may be used, for example, to determine which sample and/or samples in a readback signal correspond to the data stored on the medium. When conventional timing recovery makes an incorrect decision, the readback signal may be misinterpreted. During OPC reading operations it is particularly important to correctly synchronize with the readback signal so that calibration measurements are accurate. Therefore, it is also desirable to provide for effective correction of a readback signal (e.g., reduction of jitter in time and/or amplitude) while reading data that may have been written under a variety of conditions (e.g., calibration pattern data written to the optical storage medium using several different writing power levels, timing offsets, etc.).
In a related exemplary OPC system, as described in U.S. patent application Ser. No. 12/352,950, filed Jan. 13, 2009 (the relevant portions of which are incorporated herein by reference), the timing recovery loop takes advantage of the predetermined readback pattern and of the relative stability of low-frequency transitions (e.g., transitions between long run-lengths such as a 6T/6T transition) in order to recover the timing of the readback signal. Thus, the predetermined data may include “guide edges” with long run-lengths. When the OPC system detects a timing offset in one of these guide edges, the offset may be used to adjust the readback timing. For example, the timing offset or a derivative thereof may be provided as an error input to a conventional phase-locked loop (PLL).
PLLs generally do not react instantaneously to offset errors. As a result, the timing offsets of both guide edges and non-guide edges may be affected by PLL drift and/or other artifacts of the timing recovery process. However, it may be desirable to precisely measure timing offsets caused by variations in the optical storage medium and/or the writing characteristics used. Therefore, it is desirable to subtract any timing offsets attributable to drift in the timing loop or to other factors that may be independent of variations in the medium and/or writing characteristics.